A light emitting diode (LED) is a semiconductor device that emits light when a current is passed through it in a forward direction. Many types of LEDs are known that emit light in various wavelengths including infra-red, visible and ultra-violet regions. Many applications for LEDs are known including as indicator lights of various colours, for use in advertising displays, and in video displays.
In the past LEDs have tended to be lower power devices that produce relatively low power outputs and have not been used for general illumination purposes. More recently, however, high-power LED devices have become known that can provide an alternative to incandescent and fluorescent light sources. LED devices produce more light per watt than incandescent light sources and may therefore be useful as energy efficient light sources, while they have a number of advantages over fluorescent light sources including being easier to dim and not requiring the use of potentially toxic and polluting elements such as mercury to create the plasma that is the source of fluorescent light.
Light emitting diodes (LEDs) have therefore emerged as promising lighting devices for the future. However, LEDs are still primarily restricted to decorative, display and signaling applications so far and have not yet entered the market for general illumination to any great extent.
In photometry, one important factor that is commonly used for comparing different lighting devices is the luminous efficacy (lumen per Watt). One major hindrance to the widespread use of LEDs in general illumination applications is that the luminous flux of LEDs decreases with the junction temperature of the LEDs. The luminous efficacy of various LEDs typically decreases by approximately 0.2% to 1% per degree Celsius rise in temperature. Due to the ageing effect, the actual degradation of luminous efficacy could be higher than this quoted figures. Accelerated aging tests show that the light output can drop by a further 45%. For aged LEDs, the efficacy degradation rate could be up to 1% per ° C. In some applications such as automobile headlights and compact lamps, the ambient temperature could be very high and the size of the heatsink is limited. The drop in luminous efficacy due to thermal problem would be serious, resulting in reduction of luminous output.
In a recent article on lighting titled “Let there be light” (Harris, Mark, IET Engineering & Technology (E&T) Magazine, Vol. 4, Issue 20, 21 Nov.-4 Dec. 2009, pp. 18-21), it was commented that, in respect of LED products, “the majority of LED A-type replacement lamps do not meet manufacturer performance claims” and that “testing reveals that these lamps produce only 10 to 60 percent of their claimed light output”. Besides the quality issues, one possible reason for such mismatch in the claimed and actual luminous performance is the understanding of the luminous efficacy figures of LED devices. LED device manufacturers usually cite high luminous efficacy figures which are only correct at a junction temperature of 25° C. In practice, luminous efficacy will decrease significantly with increasing LED junction temperature. At a junction temperature under normal operation, it is not unusual that the luminous efficacy could drop by 25% or more.
FIG. 1 shows a conventional LED. At the heart of the LED device is a light emitting semiconductor material such as InGaN though other materials will be known to those skilled in the art. In the example of FIG. 1 a light-emitting InGaN chip 1 is mounted on a silicon substrate 2 and is connected to electrodes such as cathode 3 through gold wires 4 and solder connection 5. The light-emitting chip 1 is covered by a silicone encapsulant 6 and a plastic lens 7.
When a LED of the type shown in FIG. 1 is used to generate light a substantial amount of heat is generated that will damage the light-emitting chip if not removed. Therefore a heat sink must be provided and beneath the light-emitting chip 1 is a heatsink slug 2. In practice when used to provide a source of light for illumination, conventionally multiple LEDs are provided to form a LED system as shown in FIG. 2 where multiple LEDs 10 are provided on a single heatsink 11.